Copper-zinc alloy electroplating bath and plating method using the copper-zinc alloy electroplating bath

ABSTRACT

A cyanide-free copper-zinc alloy electroplating bath is provided which can form a uniform and glossy alloy layer having the desired composition even at a higher current density than that for a conventional electroplating bath, and which is excellent in productivity. 
     The copper-zinc alloy electroplating bath comprises at least one selected from a copper salt, zinc salt, alkali metal pyrophosphate, and amino acid or a salt thereof, and has a pH of 8.5 to 14. The pH is preferably 10.5 to 11.8; and the concentration of the amino acid or a salt thereof is preferably 0.08 mol/L to 0.22 mol/L, more preferably 0.1 mol/L to 0.13 mol/L. As the amino acid or a salt thereof, histidine or a salt thereof may be preferably used.

TECHNICAL FIELD

The present invention relates to a copper-zinc alloy electroplating bath and a plating method using it; more particularly, a cyanide-free copper-zinc alloy electroplating bath which can form a glossy and uniform alloy layer even at a higher current density, and a plating method using it.

BACKGROUND ART

At present, copper-zinc alloy plating is industrially widely used as decorative plating to give a brass-colored metallic luster and tone to metal products, plastic products, ceramic products and the like. However, since a conventional plating bath contains a large amount of cyanide, its toxicity has become a big problem, and the burden of disposal of cyanide-containing waste has been large.

As means for solving these problems, a number of methods for copper-zinc alloy plating wherein no cyanide is used have been reported up to now. For example, sequential plating is a practical method for application of brass plating to a product to be plated, and in such a method, a copper-plated layer and a zinc-plated layer are sequentially plated on the surface of the product to be plated by electrodeposition, followed by a thermal diffusion step. In the case of sequential brass plating, a pyrophosphate copper plating solution and an acidic zinc sulfate plating solution are usually used (e.g., Patent Document 1).

On the other hand, as a method for simultaneous plating with copper-zinc, a cyanide-free copper-zinc plating bath has also been reported, and a plating bath using a glucoheptonate bath or a potassium pyrophosphate bath supplemented with histidine as a complexing agent has been proposed (e.g., Patent Document 2).

Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-98496

Patent Document 2: Japanese Examined Patent Application Publication No. 3-20478

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, sequential plating as described in Patent Document 1 has a drawback in that it requires a number of processing steps such as a copper-plated layer formation step, zinc-plated layer formation step and thermal diffusion step, and is hence complicated, so that the operating efficiency is poor. Further, in the copper-zinc alloy electroplating bath described in Patent Document 2, although there is no problem of toxicity which arises when a bath using cyanide is employed, the current density at which a glossy and uniform alloy layer can be formed is not more than 5 A/dm² which is less than the current density required for formation of an alloy layer with high productivity, which is problematic. In either case, at present, it is difficult to put a cyanide-free copper-zinc alloy plating bath to practical use.

Thus, the present invention aims to provide a cyanide-free copper-zinc alloy electroplating bath which is capable of forming a glossy and uniform alloy layer having the desired composition even at a higher current density than that for a conventional electroplating bath with high productivity, and a plating method using it.

Means for Solving the Problems

To solve the above-described problems, the present inventors intensively studied to discover that, in a copper-zinc alloy electroplating bath comprising at least one selected from an alkali metal pyrophosphate and an amino acid or a salt thereof, a glossy and uniform alloy layer can be obtained by adjusting pH of the copper-zinc alloy electroplating bath at a current density ranging from a low current density to a high current density, thereby completing the present invention.

That is, the copper-zinc alloy electroplating bath of the present invention comprises at least one selected from a copper salt, zinc salt, alkali metal pyrophosphate, and amino acid or a salt thereof; and has a pH of 8.5 to 14.

In the copper-zinc alloy electroplating bath of the present invention, pH is preferably 10.5 to 11.8; and the concentration of the amino acid or a salt thereof is preferably 0.08 mol/L to 0.22 mol/L; and the concentration of the amino acid or a salt thereof is more preferably 0.1 mol/L to 0.13 mol/L. The total concentration of copper and zinc contained in the copper-zinc alloy electroplating bath is preferably 0.03 to 0.3 mol/L; at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides is preferably contained; and the amino acid or a salt thereof is preferably histidine or a salt thereof.

The method of copper-zinc alloy electroplating of the present invention comprises using the copper-zinc alloy electroplating bath, wherein the cathode current density is higher than 5 A/dm² and not higher than 10 A/dm².

Effect of the Invention

According to the present invention, by employing the above-described constitution, a cyanide-free copper-zinc alloy electroplating bath which is capable of forming at a current density ranging from a low current density to a high current density a uniform and glossy alloy layer having the desired composition and is capable of utilizing a higher current density than that for a conventional electroplating bath can be realized, thereby enhancing the productivity.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described in detail.

The copper-zinc alloy electroplating bath of the present invention comprises at least one selected from a copper salt, zinc salt, alkali metal pyrophosphate, and amino acid or a salt thereof; and has a pH adjusted within the range of 8.5 to 14.

As the copper salt, any one can be employed as long as it is known as a copper ion source for plating baths, and examples thereof include copper pyrophosphate, copper sulfate, cupric chloride, copper sulfamate, cupric acetate, basic copper carbonate, cupric bromide, copper formate, copper hydroxide, cupric oxide, copper phosphate, copper silicofluoride, copper stearate and cupric citrate, and either only one of these or two or more of these may be used.

As the zinc salt, any one can be employed as long as it is known as a zinc ion source for plating baths, and examples thereof include zinc pyrophosphate, zinc sulfate, zinc chloride, zinc sulfamate, zinc oxide, zinc acetate, zinc bromide, basic zinc carbonate, zinc oxalate, zinc phosphate, zinc silicofluoride, zinc stearate and zinc lactate, and either only one of these or two or more of these may be used.

The total concentration of copper and zinc dissolved in the plating bath is preferably within the range of 0.03 to 0.30 mol/L. When the concentration is lower than 0.03 mol/L, deposition of copper is predominant and hence a good alloy layer can hardly be obtained. On the other hand, when the concentration is higher than 0.30 mol/L, gloss cannot be obtained on the surface of the plated coating.

As the alkali metal pyrophosphate, any one can be employed as long as it is known, and examples thereof include sodium salt and potassium salt thereof.

It is important for the copper-zinc alloy electroplating bath of the present invention to have a pH within the range of 8.5 to 14, preferably 10.5 to 11.8. When the pH is lower than 8.5, a glossy and uniform alloy layer cannot be obtained, while when the pH is higher than 14, the current efficiency decreases. Adjustment of pH of the copper-zinc alloy electroplating bath of the present invention can be preferably carried out with an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide; or an alkaline earth metal hydroxide such as calcium hydroxide; preferably potassium hydroxide.

The concentration of the amino acid or a salt thereof of the copper-zinc alloy electroplating bath of the present invention is within the range of 0.08 mol/L to 0.22 mol/L, preferably 0.1 mol/L to 0.13 mol/L. When the concentration of the amino acid or a salt thereof is lower than 0.08 mol/L, a uniform alloy layer cannot be obtained at a high current density, while when the concentration of the amino acid or a salt thereof is higher than 0.22 mol/L, copper predominates in the composition of the alloy layer, so that a uniform alloy layer having the desired composition cannot be obtained.

As the amino acid, any one can be employed as long as it is known, and examples thereof include u-amino acids such as glycine, alanine, glutamic acid, aspartic acid, threonine, serine, proline, tryptophan and histidine, and hydrochlorides and sodium salts thereof. Histidine is preferred. Either only one of these or two or more of these may be used.

The amount of each of the above-described components to be added in the present invention is not limited and may be selected appropriately. In consideration of industrial usage, the amount of the copper salt is preferably about 2 to 40 g/L in terms of copper; the amount of the zinc salt is preferably about 0.5 to 30 g/L in terms of zinc; the amount of the alkali metal pyrophosphate is preferably about 150 to 400 g/L; and the amount of the amino acid or a salt thereof is preferably about 0.2 to 50 g/L.

In the plating method using a copper-zinc alloy electroplating bath of the present invention, the copper-zinc alloy electroplating bath of the present invention is employed, and plating is carried out at a high current density higher than 5 A/dm² and not higher than 10 A/dm². When copper-zinc alloy electroplating is carried out using the copper-zinc alloy electroplating bath of the present invention, a conventional electroplating method may be employed. For example, the electroplating may be carried out at a bath temperature of about 30 to 40° C. without stirring, with mechanical stirring or with air agitation. In this case, as the anode, any one may be used as long as it is one used for conventional electroplating of a copper-zinc alloy. By employing the copper-zinc alloy electroplating bath of the present invention, it is possible to carry out plating at a high current density higher than 5 A/dm² and not higher than 10 A/dm², so that a glossy and uniform copper-zinc alloy layer can be formed with higher productivity than before.

Before carrying out the electroplating, the material to be plated may be subjected to conventional pretreatment such as buffing, degreasing, and soaking in a dilute acid according to conventional methods, and an undercoat such as gloss nickel plating may be also applied to the material. After the plating, a conventional operation such as washing with water, washing with hot water or drying may be carried out, and soaking in a dichromic acid dilute solution, clear painting or the like may be further carried out as required.

In the present invention, the material to be plated is not limited, and any one to which a copper-zinc alloy electroplating coat can be usually applied may be used, and examples thereof include metal products such as steel filaments used in steel cords for reinforcing rubber articles; plastic products; and ceramic products.

EXAMPLES

The present invention will be described in more detail by way of Examples.

According to the composition of the copper-zinc alloy electroplating bath shown in each of Tables 1 to 3 below, the copper-zinc alloy electroplating bath of each Example was prepared, and according to the plating conditions in Tables 1 to 3, copper-zinc alloy electroplating was carried out. After preparation of each plating bath, plating was immediately carried out, and the deposited amount of plating and the alloy composition were analyzed. The roughness of the alloy surface was observed by a laser microscope to obtain the roughness parameters Ra, Rv and Rz. Further, the range of current density within which a glossy and uniform alloy layer can be obtained was calculated. Tables 1 to 3 below also show the obtained results.

<Ra>

Ra was calculated according to

${Ra} = {\frac{1}{L}{\int_{0}^{L}{{{f(x)}}{x}}}}$

which is the centerline average roughness (Ra) of the surface of the material to be plated. For calculation of the centerline average roughness, a portion of the roughness profile, which portion had a measurement length L in the direction of its centerline, was sampled, and the centerline of the sampled portion was taken along the x-axis and the longitudinal magnification was taken along the y-axis to represent the roughness profile as y=f(x). The value Ra given by the above equation was represented in micrometers (μm).

<Rv>

For calculation of the maximum valley depth (Rv), a portion of the roughness profile, which portion had a measurement length L in the direction of its centerline, was sampled, and the maximum value of the valley depth Zv of the roughness profile was represented in micrometers (μm).

<Rz>

For calculation of the maximum height of the roughness (Rz), a portion of the roughness profile, which portion had a measurement length L in the direction of its centerline, was sampled, and the value represented by the sum of the maximum value of peak height Zp and the maximum value of valley depth Zv of the roughness profile was represented in micrometers (μm).

TABLE 1 Example 1 Example 2 Example 3 Example 4 Plating Copper sulfate 25.1 25.1 25.1 25.1 bath pentahydrate (g/L) (0.10) (0.10) (0.10) (0.10) composition (mol/L) Zinc sulfate heptahydrate (g/L) 20.2 20.2 20.2 20.2 (mol/L) (0.07) (0.07) (0.07) (0.07) Potassium pyrophosphate (g/L) 347.7 347.7 152.13 347.7 (mol/L) (1.0) (1.0) (0.44) (1.0) Sodium pyrophosphate — — — — decahydrate (g/L) (mol/L) L-histidine (g/L) 2.1 0.21 2.1 2.1 (mol/L) (0.01) (0.001) (0.01) (0.01) Potassium hydroxide Added for Added for Added for — adjusting adjusting adjusting pH pH pH Sodium hydroxide — — — — pH 11.6 10.3 11.1 9.2 Plating Bath temperature (° C.) 30 30 30 30 condition Cathode current density (A/dm²) 0.1-15 0.1-15 0.1-15 0.1-15 Plating time (sec) 120 120 120 120 Evaluation Deposited amount of plating* 0.288 0.264 0.293 0.302 (mg/cm²) Composition of plated coating* 62.8 64.1 63.5 66.0 (Cu % by mass) Roughness parameter Ra* (μm) 0.60 0.82 0.73 0.49 Roughness parameter Rv*(μm) 1.53 1.58 1.44 1.44 Roughness parameter Rz*(μm) 2.57 2.43 2.35 2.31 Gloss/uniformity-achieving   3-10  3-6   3-10  3-5 current density range (A/dm²) *Evaluation results at a cathode current density of 3 A/dm²

TABLE 2 Example Example Example 5 6 7 Plating Copper sulfate pentahydrate (g/L) 25.1 25.1 6.28 bath (mol/L) (0.10) (0.10) (0.03) composition Zinc sulfate heptahydrate (g/L) 20.2 20.2 5.05 (mol/L) (0.07) (0.07) (0.02) Potassium pyrophosphate (g/L) — — — (mol/L) Sodium pyrophosphate 183.0 137.25 68.63 decahydrate (g/L) (0.41) (0.31) (0.15) (mol/L) L-histidine (g/L) 15.67 15.67 3.91 (mol/L) (0.1) (0.1) (0.03) Potassium hydroxide Added for Added for — adjusting adjusting pH pH Sodium hydroxide — — Added for adjusting pH pH 13.5 9.5 11.0 Plating Bath temperature (° C.) 30 30 30 condition Cathode current density (A/dm²) 0.1-15 0.1-15 0.1-15 Plating time (sec) 120 120 120 Evaluation Deposited amount of plating* 0.300 0.277 0.269 (mg/cm²) Composition of plated coating* 65.0 64.4 66.8 (Cu % by mass) Roughness parameter Ra* (μm) 0.26 0.22 0.18 Roughness parameter Rv* (μm) 0.60 0.58 0.68 Roughness parameter Rz* (μm) 1.12 1.01 1.03 Gloss/uniformity-achieving current   1-10  1-6  3-6 density range (A/dm²) *Evaluation results at a cathode current density of 3 A/dm²

TABLE 3 Example Example Example 8 Example 9 10 11 Plating Copper sulfate 25.1 25.1 25.1 25.1 bath pentahydrate (g/L) (0.10) (0.10) (0.10) (0.10) composition (mol/L) Zinc sulfate heptahydrate (g/L) 20.2 20.2 20.2 20.2 (mol/L) (0.07) (0.07) (0.07) (0.07) Potassium pyrophosphate (g/L) 250.0 152.13 347.7 347.7 (mol/L) (0.72) (0.44) (1.0) (1.0) Sodium pyrophosphate — — — — decahydrate (g/L) (mol/L) L-histidine (g/L) 21.2 21.2 42.4 2.1 (mol/L) (0.1) (0.1) (0.2) (0.01) Potassium hydroxide Added for Added for — — adjusting adjusting pH pH Sodium hydroxide — — — — pH 11.4 11.1 9.0 9.7 Plating Bath temperature (° C.) 30 30 30 30 condition Cathode current density (A/dm²) 0.1-15 0.1-15 0.1-15 0.1-15 Plating time (sec) 120 120 120 120 Evaluation Gloss/uniformity-achieving   4-13   4-10  4-8  3-5 current density range (A/dm²)

By comparison of the results in Examples 1 to 11 in the above Tables, it was confirmed that, by adjusting pH of the plating bath within the range of 8.5 to 14, the range of the current density within which a glossy and uniform alloy layer can be formed was extended in the direction of the high-current-density side. 

1. A copper-zinc alloy electroplating bath comprising at least one selected from a copper salt, zinc salt, alkali metal pyrophosphate, and amino acid or a salt thereof, said copper-zinc alloy electroplating bath having a pH of 8.5 to
 14. 2. The copper-zinc alloy electroplating bath according to claim 1, wherein said pH is 10.5 to 11.8.
 3. The copper-zinc alloy electroplating bath according to claim 1, wherein the concentration of said amino acid or a salt thereof is 0.08 mol/L to 0.22 mol/L.
 4. The copper-zinc alloy electroplating bath according to claim 1, wherein the concentration of said amino acid or a salt thereof is 0.1 mol/L to 0.13 mol/L.
 5. The copper-zinc alloy electroplating bath according to claim 1, wherein the sum of the concentrations of copper and zinc contained in said copper-zinc alloy electroplating bath is within the range of 0.03 to 0.3 mol/L.
 6. The copper-zinc alloy electroplating bath according to claim 1, comprising at least one selected from an alkali metal hydroxide and an alkaline earth metal hydroxide.
 7. The copper-zinc alloy electroplating bath according to claim 1, wherein said amino acid or a salt thereof is histidine or a salt thereof.
 8. A electroplating method of copper-zinc alloy using the copper-zinc alloy electroplating bath according to claim 1, wherein the cathode current density in the plating bath is more than 5 A/dm² and not more than 10 A/dm². 